1. Field of the invention
The invention concerns a contact lens with spherical aberration correction for presbyopia.
2. Description of the prior art
The spherical aberration of a lens (whether that of the human eye or a contact lens placed on the cornea) is generally characterized in that the position of the point of intersection of the emerging ray and the optical axis of the lens depends on the height h at which the incident ray passes through said lens. A patch is then obtained on the retina rather than a point image. This reduces the discrimination of two points at a great distance. This reduces visual performance which causes visual discomfort.
Physiological studies show that the spherical aberration of the human eye increases with age.
U.S. Pat. No. 3,778,937 ("Method for making aspheric surfaces" Neefe, Nov. 1, 1971), describes a method of selectively abrading a portion of the outside face of a unifocal contact lens to confer on it a spherical surface which can correct the spherical aberration of the eye.
U.S. Pat. No. 4,564,484 ("Production of soft lenses having reduced spherical aberrations", Neefe, Nov. 26, 1984), granted to the same inventor, discloses a method of fabrication characterized by the production of unifocal lenses whose outside face is initially a spherical dome and whose inside face is shaped to have a radius of curvature that is progressively reduced toward the edge of the lens. When the lens is placed on the eye its internal face espouses the spherical contour of the cornea and it therefore has a radius of curvature of the outside face that progressively increases toward the edge of the lens, which enables correction of spherical aberration.
In the particular case of persons suffering from presbyopia, different corrections are required in concentric areas within the pupil. Bifocal or multifocal lenses having a plurality of separate correction areas are conventionally used to correct presbyopia. They generally have a discontinuity of power between the different correction areas corresponding to near and far vision.
Application FR 89 01417 ("Lentille optique a vision simultanee pour la correction de la presbytie" ["Simultaneous vision optical lens for correcting presbyopia"], Essilor, Feb. 3, 1989), publication number 2 642 854, describes a progressive simultaneous vision optical lens for correcting presbyopia defined by the curve representing its power as a function of the height relative to the axis. This curve is inscribed within an area contained between a lower envelope curve and an upper envelope curve which satisfy polynomial equations determined for this purpose and define two separate vision areas, one corresponding to far vision and the other corresponding to near vision. In practice there are one or more continuous intermediate vision areas between the far and near vision areas.
The skilled person can determine the surfaces to be imparted to the inside and outside faces of the optical lens from a polynomial equation so that it satisfies the corresponding power conditions.
As specified in the above French patent, to which reference may usefully be made, the term sagittal power is used to denote the reciprocal, in diopters, of the distance in meters measured from the main plane of the optical lens at which any light ray parallel to the optical axis of the lens and at a height h above the axis intersects the axis after passing through the optical lens.
The lens for correcting then has two main areas, one of which corresponds to near vision and the other to far vision, these areas being concentric and linked by an area of continuous evolution. In practice the near vision power is deduced from the far vision power by adding a particular "addition" characteristic of the presbyopia to be corrected.
In French patent 93 01831, publication number 2 701 770 ("Lentille ophtalmique a vision simultanee pour la correction de la presbytie et jeux de deux telles lentilles ophtalmiques pour un meme porteur" ["Simultaneous vision ophthalmic lens for correcting presbyopia and set of two such ophthalmic lenses for the same wearer"], Essilor, Feb. 18, 1993), there are particular provisions to give preference to either the far or near vision area over the other, on the basis of lenses of the kind in which the power curve is between two polynomial envelopes.
In a particular case of weak presbyopia, characterized by a low value of the addition (less than 1.5 diopters), French patent 95 09016 ("Lentille optique a vision simultanee progressive pour la correction d'une presbytie correspondant a une faible addition" ["Progressive simultaneous vision optical lens for correcting presbyopia corresponding to a low addition"], Essilor, Jul. 25, 1995) proposes a lens of the kind in which the power curve is between two polynomial envelopes and in which the intermediate area between the near vision and far vision areas is such that the near vision area is relatively large compared to lenses with a high addition.
The problem of correcting spherical aberration, touched on above, is naturally more acute for persons suffering from presbyopia or wearing lenses with a high correcting power.
The article by T. C. A. Jenkins, "Aberrations of the eye and their effects on vision", published in the British Journal of Physiological Optics 20 p. 59-91, 1963 shows that the spherical aberration of the eye varies with age.
PCT application WO 94/23327 ("Contact lens designed to accommodate and correct for the effects of presbyopia", Pilkington Mar. 22, 1994) describes a type of lens for correcting presbyopia including two concentric areas, the central, "direct" vision area being corrected for spherical aberration by a parabolic or hyperbolic internal shape (before fitting to the eye) and the peripheral area, applying a correction different to that of the central area and corresponding to "indirect" vision, having a spherical surface.
The invention is directed to a progressive multifocal ophthalmic lens, the design of which allows for the evolution of ocular spherical aberration with age and for its relation to ametropia.